.TH CJPEG 1 "4 November 2020"

.SH NAME

cjpeg \- compress an image file to a JPEG file

.SH SYNOPSIS

.B cjpeg

[

.I options

]

[

.I filename

]

.LP

.SH DESCRIPTION

.LP

.B cjpeg

compresses the named image file, or the standard input if no file is

named, and produces a JPEG/JFIF file on the standard output.

The currently supported input file formats are: PPM (PBMPLUS color

format), PGM (PBMPLUS grayscale format), BMP, Targa, and RLE (Utah Raster

Toolkit format).  (RLE is supported only if the URT library is available.)

.SH OPTIONS

All switch names may be abbreviated; for example,

.B \-grayscale

may be written

.B \-gray

or

.BR \-gr .

Most of the "basic" switches can be abbreviated to as little as one letter.

Upper and lower case are equivalent (thus

.B \-BMP

is the same as

.BR \-bmp ).

British spellings are also accepted (e.g.,

.BR \-greyscale ),

though for brevity these are not mentioned below.

.PP

The basic switches are:

.TP

.BI \-quality " N[,...]"

Scale quantization tables to adjust image quality.  Quality is 0 (worst) to

100 (best); default is 75.  (See below for more info.)

.TP

.B \-grayscale

Create monochrome JPEG file from color input.  Be sure to use this switch when

compressing a grayscale BMP file, because

.B cjpeg

isn't bright enough to notice whether a BMP file uses only shades of gray.

By saying

.BR \-grayscale,

you'll get a smaller JPEG file that takes less time to process.

.TP

.B \-rgb

Create RGB JPEG file.

Using this switch suppresses the conversion from RGB

colorspace input to the default YCbCr JPEG colorspace.

.TP

.B \-optimize

Perform optimization of entropy encoding parameters.  Without this, default

encoding parameters are used.

.B \-optimize

usually makes the JPEG file a little smaller, but

.B cjpeg

runs somewhat slower and needs much more memory.  Image quality and speed of

decompression are unaffected by

.BR \-optimize .

.TP

.B \-progressive

Create progressive JPEG file (see below).

.TP

.B \-targa

Input file is Targa format.  Targa files that contain an "identification"

field will not be automatically recognized by

.BR cjpeg ;

for such files you must specify

.B \-targa

to make

.B cjpeg

treat the input as Targa format.

For most Targa files, you won't need this switch.

.PP

The

.B \-quality

switch lets you trade off compressed file size against quality of the

reconstructed image: the higher the quality setting, the larger the JPEG file,

and the closer the output image will be to the original input.  Normally you

want to use the lowest quality setting (smallest file) that decompresses into

something visually indistinguishable from the original image.  For this

purpose the quality setting should generally be between 50 and 95 (the default

is 75) for photographic images.  If you see defects at

.B \-quality

75, then go up 5 or 10 counts at a time until you are happy with the output

image.  (The optimal setting will vary from one image to another.)

.PP

.B \-quality

100 will generate a quantization table of all 1's, minimizing loss in the

quantization step (but there is still information loss in subsampling, as well

as roundoff error.)  For most images, specifying a quality value above

about 95 will increase the size of the compressed file dramatically, and while

the quality gain from these higher quality values is measurable (using metrics

such as PSNR or SSIM), it is rarely perceivable by human vision.

.PP

In the other direction, quality values below 50 will produce very small files

of low image quality.  Settings around 5 to 10 might be useful in preparing an

index of a large image library, for example.  Try

.B \-quality

2 (or so) for some amusing Cubist effects.  (Note: quality

values below about 25 generate 2-byte quantization tables, which are

considered optional in the JPEG standard.

.B cjpeg

emits a warning message when you give such a quality value, because some

other JPEG programs may be unable to decode the resulting file.  Use

.B \-baseline

if you need to ensure compatibility at low quality values.)

.PP

The \fB-quality\fR option has been extended in this version of \fBcjpeg\fR to

support separate quality settings for luminance and chrominance (or, in

general, separate settings for every quantization table slot.)  The principle

is the same as chrominance subsampling:  since the human eye is more sensitive

to spatial changes in brightness than spatial changes in color, the chrominance

components can be quantized more than the luminance components without

incurring any visible image quality loss.  However, unlike subsampling, this

feature reduces data in the frequency domain instead of the spatial domain,

which allows for more fine-grained control.  This option is useful in

quality-sensitive applications, for which the artifacts generated by

subsampling may be unacceptable.

.PP

The \fB-quality\fR option accepts a comma-separated list of parameters, which

respectively refer to the quality levels that should be assigned to the

quantization table slots.  If there are more q-table slots than parameters,

then the last parameter is replicated.  Thus, if only one quality parameter is

given, this is used for both luminance and chrominance (slots 0 and 1,

respectively), preserving the legacy behavior of cjpeg v6b and prior.

More (or customized) quantization tables can be set with the \fB-qtables\fR

option and assigned to components with the \fB-qslots\fR option (see the

"wizard" switches below.)

.PP

JPEG files generated with separate luminance and chrominance quality are fully

compliant with standard JPEG decoders.

.PP

.BR CAUTION:

For this setting to be useful, be sure to pass an argument of \fB-sample 1x1\fR

to \fBcjpeg\fR to disable chrominance subsampling.  Otherwise, the default

subsampling level (2x2, AKA "4:2:0") will be used.

.PP

The

.B \-progressive

switch creates a "progressive JPEG" file.  In this type of JPEG file, the data

is stored in multiple scans of increasing quality.  If the file is being

transmitted over a slow communications link, the decoder can use the first

scan to display a low-quality image very quickly, and can then improve the

display with each subsequent scan.  The final image is exactly equivalent to a

standard JPEG file of the same quality setting, and the total file size is

about the same --- often a little smaller.

.PP

Switches for advanced users:

.TP

.B \-arithmetic

Use arithmetic coding.

.B Caution:

arithmetic coded JPEG is not yet widely implemented, so many decoders will be

unable to view an arithmetic coded JPEG file at all.

.TP

.B \-dct int

Use accurate integer DCT method (default).

.TP

.B \-dct fast

Use less accurate integer DCT method [legacy feature].

When the Independent JPEG Group's software was first released in 1991, the

compression time for a 1-megapixel JPEG image on a mainstream PC was measured

in minutes.  Thus, the \fBfast\fR integer DCT algorithm provided noticeable

performance benefits.  On modern CPUs running libjpeg-turbo, however, the

compression time for a 1-megapixel JPEG image is measured in milliseconds, and

thus the performance benefits of the \fBfast\fR algorithm are much less

noticeable.  On modern x86/x86-64 CPUs that support AVX2 instructions, the

\fBfast\fR and \fBint\fR methods have similar performance.  On other types of

CPUs, the \fBfast\fR method is generally about 5-15% faster than the \fBint\fR

method.

For quality levels of 90 and below, there should be little or no perceptible

quality difference between the two algorithms.  For quality levels above 90,

however, the difference between the \fBfast\fR and \fBint\fR methods becomes

more pronounced.  With quality=97, for instance, the \fBfast\fR method incurs

generally about a 1-3 dB loss in PSNR relative to the \fBint\fR method, but

this can be larger for some images.  Do not use the \fBfast\fR method with

quality levels above 97.  The algorithm often degenerates at quality=98 and

above and can actually produce a more lossy image than if lower quality levels

had been used.  Also, in libjpeg-turbo, the \fBfast\fR method is not fully

accelerated for quality levels above 97, so it will be slower than the

\fBint\fR method.

.TP

.B \-dct float

Use floating-point DCT method [legacy feature].

The \fBfloat\fR method does not produce significantly more accurate results

than the \fBint\fR method, and it is much slower.  The \fBfloat\fR method may

also give different results on different machines due to varying roundoff

behavior, whereas the integer methods should give the same results on all

machines.

.TP

.BI \-icc " file"

Embed ICC color management profile contained in the specified file.

.TP

.BI \-restart " N"

Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is

attached to the number.

.B \-restart 0

(the default) means no restart markers.

.TP

.BI \-smooth " N"

Smooth the input image to eliminate dithering noise.  N, ranging from 1 to

100, indicates the strength of smoothing.  0 (the default) means no smoothing.

.TP

.BI \-maxmemory " N"

Set limit for amount of memory to use in processing large images.  Value is

in thousands of bytes, or millions of bytes if "M" is attached to the

number.  For example,

.B \-max 4m

selects 4000000 bytes.  If more space is needed, an error will occur.

.TP

.BI \-outfile " name"

Send output image to the named file, not to standard output.

.TP

.BI \-memdst

Compress to memory instead of a file.  This feature was implemented mainly as a

way of testing the in-memory destination manager (jpeg_mem_dest()), but it is

also useful for benchmarking, since it reduces the I/O overhead.

.TP

.B \-verbose

Enable debug printout.  More

.BR \-v 's

give more output.  Also, version information is printed at startup.

.TP

.B \-debug

Same as

.BR \-verbose .

.TP

.B \-version

Print version information and exit.

.PP

The

.B \-restart

option inserts extra markers that allow a JPEG decoder to resynchronize after

a transmission error.  Without restart markers, any damage to a compressed

file will usually ruin the image from the point of the error to the end of the

image; with restart markers, the damage is usually confined to the portion of

the image up to the next restart marker.  Of course, the restart markers

occupy extra space.  We recommend

.B \-restart 1

for images that will be transmitted across unreliable networks such as Usenet.

.PP

The

.B \-smooth

option filters the input to eliminate fine-scale noise.  This is often useful

when converting dithered images to JPEG: a moderate smoothing factor of 10 to

50 gets rid of dithering patterns in the input file, resulting in a smaller

JPEG file and a better-looking image.  Too large a smoothing factor will

visibly blur the image, however.

.PP

Switches for wizards:

.TP

.B \-baseline

Force baseline-compatible quantization tables to be generated.  This clamps

quantization values to 8 bits even at low quality settings.  (This switch is

poorly named, since it does not ensure that the output is actually baseline

JPEG.  For example, you can use

.B \-baseline

and

.B \-progressive

together.)

.TP

.BI \-qtables " file"

Use the quantization tables given in the specified text file.

.TP

.BI \-qslots " N[,...]"

Select which quantization table to use for each color component.

.TP

.BI \-sample " HxV[,...]"

Set JPEG sampling factors for each color component.

.TP

.BI \-scans " file"

Use the scan script given in the specified text file.

.PP

The "wizard" switches are intended for experimentation with JPEG.  If you

don't know what you are doing, \fBdon't use them\fR.  These switches are

documented further in the file wizard.txt.

.SH EXAMPLES

.LP

This example compresses the PPM file foo.ppm with a quality factor of

60 and saves the output as foo.jpg:

.IP

.B cjpeg \-quality

.I 60 foo.ppm

.B >

.I foo.jpg

.SH HINTS

Color GIF files are not the ideal input for JPEG; JPEG is really intended for

compressing full-color (24-bit) images.  In particular, don't try to convert

cartoons, line drawings, and other images that have only a few distinct

colors.  GIF works great on these, JPEG does not.  If you want to convert a

GIF to JPEG, you should experiment with

.BR cjpeg 's

.B \-quality

and

.B \-smooth

options to get a satisfactory conversion.

.B \-smooth 10

or so is often helpful.

.PP

Avoid running an image through a series of JPEG compression/decompression

cycles.  Image quality loss will accumulate; after ten or so cycles the image

may be noticeably worse than it was after one cycle.  It's best to use a

lossless format while manipulating an image, then convert to JPEG format when

you are ready to file the image away.

.PP

The

.B \-optimize

option to

.B cjpeg

is worth using when you are making a "final" version for posting or archiving.

It's also a win when you are using low quality settings to make very small

JPEG files; the percentage improvement is often a lot more than it is on

larger files.  (At present,

.B \-optimize

mode is always selected when generating progressive JPEG files.)

.SH ENVIRONMENT

.TP

.B JPEGMEM

If this environment variable is set, its value is the default memory limit.

The value is specified as described for the

.B \-maxmemory

switch.

.B JPEGMEM

overrides the default value specified when the program was compiled, and

itself is overridden by an explicit

.BR \-maxmemory .

.SH SEE ALSO

.BR djpeg (1),

.BR jpegtran (1),

.BR rdjpgcom (1),

.BR wrjpgcom (1)

.br

.BR ppm (5),

.BR pgm (5)

.br

Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",

Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.

.SH AUTHOR

Independent JPEG Group

.PP

This file was modified by The libjpeg-turbo Project to include only information

relevant to libjpeg-turbo, to wordsmith certain sections, and to describe

features not present in libjpeg.

.SH ISSUES

Support for GIF input files was removed in cjpeg v6b due to concerns over

the Unisys LZW patent.  Although this patent expired in 2006, cjpeg still

lacks GIF support, for these historical reasons.  (Conversion of GIF files to

JPEG is usually a bad idea anyway, since GIF is a 256-color format.)

.PP

Not all variants of BMP and Targa file formats are supported.

.PP

The

.B \-targa

switch is not a bug, it's a feature.  (It would be a bug if the Targa format

designers had not been clueless.)